1. Field of the Invention
This invention relates generally to methods and apparatus for sorbing organic substances onto a media and desorbing the organic substances from the media.
2. Discussion of the Prior Art
Industrial ventilation air is often laden with organic substances, such as solvents, which are commonly referred to as volatile organic compounds or VOCs. In the interests of purifying this ventilation air, it is desirable to remove the VOCs from the ventilation air. As taught by Dingfors et al, in U.S. Pat. No. 4,902,311, which is incorporated herein by reference, the VOCs can be adsorbed onto microporous polymeric particles. These particles can then be fed into a desorption chamber in which the solvent is desorbed from the adsorption media, stripped off and condensed in a cooler.
The desorption is typically accomplished by heating the media, for example with microwaves, as taught by Opperman in U.S. Pat. No. 5,509,956, which is incorporated herein by reference. Opperman teaches that the VOC-laden adsorption media can be contained in a cylindrical vessel with microwaves introduced down an axial waveguide and radiated outwardly through the adsorption media. This structure tends to facilitate uniform heating throughout the adsorption media. Nevertheless, in this batch process, the microwave energy tends to form standing waves which have peaks of high energy and valleys of low energy. For this reason, even the symmetrical structure of the cylindrical container tends to develop hot and cold spots within the batch media.
When the adsorption media is presented with a significant depth, it has been found that the microwave energy tends to create a temperature front that travels outwardly through the depth of the media. For example in a normal batch process, the adsorption media may have a thickness such as eight inches through which the microwave energy must pass. As the temperature front contacts one bead, its VOCs are driven off and adsorbed by the next outwardly-adjacent bead. When the front contacts the second bead those VOCs are driven off to the third bead where they are first adsorbed and then ultimately desorbed. This process, commonly referred to as progressive adsorption/desorption, requires a significant period of time as the temperature front passes radially outwardly through the depth of the media.
Adsorption processes of the past have also contained the media in fluidized beds. These beds are typically stacked and the contaminated air directed progressively through the stack. A high velocity of air is required to percolate through the beads in this adsorption process. The fluidized beds may be angled slightly to promote translation of the beads among the beds. Nevertheless, the process is substantially static as it may require several days for the full translation of beads to occur.
It is always desirable to accomplish the adsorption and desorption with a minimum amount of energy and a maximum throughput of VOCs. Of particular interest is any reduction in the quantity of adsorption media required to accomplish the air purification. Since this adsorption media can be particularly expensive, large volume requirements greatly increase the cost of purification.
In accordance with the present invention, an apparatus and method is disclosed which provides for a continuous flow of the adsorption media, initially through an adsorption chamber and ultimately through a desorption chamber in a continuous closed process. The adsorption media can be introduced through an adsorber and a desorber each having a media flow shaper which maximizes the efficiency of both the adsorber and desorber based on the adsorption kinetics of the media. With a continuous flow of the media through the adsorption and desorption chambers, each particle of the media is exposed to many different levels of energy in the pattern microwave. Thus the continuous flow of the adsorption media promotes even further uniform heating of the media as it passes through the various energy levels of the pattern.
In addition, the thickness of the media is controlled and limited in order to reduce the significant delay associated with the progressive adsorption/desorption at a temperature front. For example, the media can be introduced through a flow shaper, which limits the thickness of the media flow, for example, to only one inch. Parallel plates can be used to shape this media flow, or alternatively, the media can be introduced onto the apex of a cone in order to produce a layer which progressively thins as the media spreads towards the base of the cone.
In an adsorption chamber, the media shaper can include two porous barriers or plates between which the media continuously flows and across which the VOC laden air can be introduced to the media. In an adsorption apparatus, the barriers can comprise screens or plates formed of porous materials which are penetrable by the contaminated air.
In a desorber, similar plates can shape the media flow. In a desorption apparatus, the plates can be ceramic to permit penetration of the microwave energy, and porous where it is desirable to introduce a purge gas, such as nitrogen.
In one aspect of the invention, an apparatus for removing VOCs from a gas stream flowing along a gas flow path includes an adsorber with an adsorption chamber having a first dimension along the gas flow path. A desorber receives the adsorption media with the adsorbed VOCs, and desorbs the VOCs from the adsorption media. A microwave generator provides a microwave flow path through a desorption chamber which has a second dimension along the microwave flow path. The first dimension of the adsorption chamber is greater than the second dimension of the desorption chamber.
In another aspect of the invention, a media flow distributor is adapted to control a flow of sorption media through a sorber. A media flow shaper receives the flow of sorption media and provides the flow with a restricted width not greater than a predetermined width along a particular surface. A conduit for receiving an energy stream in proximity to the media flow shaper and for directing the energy stream generally normal to the particular surface, facilitates sorption relative to the media. In a desorber the predetermined width will typically be less than one inch in order to limit progressive adsorption/desorption along a temperature front.
An adsorption system characterizes a further aspect of the invention. In this system a material is continuously adsorbed onto an adsorption media. A conduit for containing the material in a gas flow continuously directs the gas flow along a gas flow path. A media flow shaper disposed in the gas flow path is adapted to receive the media and to continuously direct the media along a media flow path transverse to the gas flow path. The media flow chamber is at least partially defined by a porous wall penetrable by the gas flow. A first valve is operable to move the media to the media flow shaper at a first throughput, while a second valve is operable to move the media from the media flow shaper at a second throughput equal to about the first throughput. The first and second valves have properties facilitating flow of the media through the valves while inhibiting flow of any gas through the valves.
In a further aspect of the invention, a sorption system is provided for sorbing a material relative to a sorption media. A source of energy, having properties for moving material relative to the adsorption media, is coupled through a conduit which directs the energy flow along an energy flow path. A media flow shaper is disposed in the energy flow path and adapted to receive the sorption media. A wall is included in the media flow shaper for directing the media flow path transverse to the energy flow path, the wall being disposed between the media flow path and the source of energy. The wall is transmissive of the energy so that the conduit directs the energy through the wall and into the media flow path to sorb the material relative to the sorption media.